Human Chimeras? (Relates to CSI 20/05, spoilers)

Tonight’s episode of CSI involved a rapist who was initially let go because his DNA sample from his epithelial cells didn’t match the DNA from the semen in the victim. They finally discover that he’s a chimera, and so his DNA is different in different tissues.

I realize that this is TV and they’re taking liberties with the truth, but I want to know just how off-base their explanation is.

They said that he was initially a fraternal twin, and “absorbed” his brother at a very early stage of development, and so now he has both sets of DNA, expressed in different tissues.

He didn’t know this till after his bone marrow donation was rejected by his leukemia-afflicted little brother. After the brother died, the doctors told this guy about his strange condition.

  1. Absorbing a fraternal twin? Is that even possible, and would it create this type of man whose semen has one set of genes, and his skin cells another, and his blood cells both? I’ve heard of twin animals having blood exchange in utero, mixing their hematopoietic stem cells and leaving their blood with 2 separate sets of blood cells. But that shouldn’t affect the rest of the tissues.

  2. Wouldn’t the tests for determining the bone marrow compatibility have picked up on the mosaicism? If both DNA sets are in the marrow (presumably the cause for the rejection), then both will be present in the blood cells… no?

I’m very very curious about all this now, so I hope someone out there knows about this stuff!

It’s incorrect to refere to the process as ‘absorption’ of the twin. The man didn’t absorb his twin, or at least no more than his twin absorbed him. Two embryos at a very early stage of development, fused into one larger embryo. Neither mebryo dies or was absorbed. It’s like pouring two glasses of water into a bucket. You wouldn’t say that one glass has abrobed the other. The two glasses have mixed and that’s all. In the same manner the two embryos mixed. Neither was absorbed. The adult human being is just one person composed of two separate cell lines. He isn’t the surviving twin since neither twin died (and arguably neither twin ever lived). He is just one person in the same way as a atrnsplant recipient is one person.

Yes it’s possible and yes it does happen. A simple Google search wil prove that. Whether it would create a person whose semen and slkin were genetically diferent is pot luck. The cells form the two embtyos are mixed randomly and if the cells that ent to form the testes were exclusoively from one embryo and the skin cell sexclusively form another then it’s possible. That would be arre though. More commonly chimaeras have mixtures of skin cells from different cell lines, ofen in ‘stripes’. Presumably many other organs will simlarly be composed of different cell lines. So some of the man’s sperm would be the same as some of his skin and some would be the same. It woul depedn on what part of his body the skin sample came from.

If both cell lines were present in the marrow then they would both be present in the blood. However that may not have been the case. The marrow at the place where it was collected for testing could easily have been derived exclusively form one cell line. The transplant however presumably harvests cells from a much larger area so has alarger chance of picking up some of the other cell line.

Didn’t see the CSI Episode, but chimerism is a real condition. Here is a case of a woman who had different DNA in here blood and ovaries.

Just watched the episode, and they did a good job with the biology. The skin striping is pretty much right out of an article I recently read about this subject. And just FYI, there have been a few threads about human chimeras on this board recently. Even one in which the CSI plot was speculated on-- ie, false DNA testing in crime cases. The interesting thing is that there isn’t any good data on how common this is. I seem to recall one article suggesting that it was a lot more common than one might think.

weird, man.


The incidence of twin births is 2-3%, but it’s speculated that perhaps one in 8 pregnancies begins as twins (either fraternal or identical).

Some times one embryo dies and is re-absorbed by the mother (“vanishing twin” syndrome), sometimes they join at an early stage.

If you think about it, if one embryo can split and become identical twins, there’s no reason why two couldn’t join and become one person.

I’m a “vanishing twin.”

There were two embryos during my mother’s pregnancy with me but only one birth.

I don’t know anything else about “absorbing” twins or chimerism or anything else because I’ve never looked inot it any further. Just thought I’d share.

This mosaicism is probably more common than we know, because unless something comes up that draws attention to it, it doesn’t cause any problems for the folks with the condition.

One of the big medical journals recently published a photo of a male/female mosaic - that is, the kid had cells with perfectly normal XX cells and perfectly normal XY cells in the same body. Aside from having skin in two distinct shades as well, the kid had extremely ambiguous genitals and was, in fact, a true hermaphrodite - now that’s a rare phenomena. In fact, it was the intersex status of the kid that had led to the picture and the discussion, along with the comment that if the genitals had not been affected (that is, had been formed of one cell line or the other instead of both) the mosaicism probably would have been undetected, even with the different skin shades.

This is fascinating stuff. How is it that the chimera’s immune system doesn’t go to town on the cell line that’s different from itself (different from the immune system’s DNA)? IANAD but I would expect severe immunity system problems here. I guess I don’t know jack!

Hey Otto, how did you find out? Surely your mom’s doctor wasn’t looking at embryos in the early stages of pregnancy.

If the answer’s too personal, that’s cool, don’t have to answer.

Basically the merging of the two embryos happens at an early stage when the immune system has not been established yet.


Don’t you mean “we”?





Sorry :smiley:

Here’s the Nature article about this topic.

Has this ever been documented in a criminal case? It calls to mind the old murder-mystery trick where the victim is stabbed in the left side of the chest… but he’s mirror-imaged, so his heart is on the right, and he survives. Yeah, it’s possible, but it’s rare, and far overdone in thriller stories.

As Monkeyboy said, the merging occurs very early on. Until the foetus is quite old the immune system is termed ‘naïve’ which means that it will accept anything that it comes into contact with as ‘self’. Naivete declines with age but even at birth the immune system is still somewhat naïve. The substances don’t even need to be human, a young enough embryo can be exposed to fish proteins, for example, and forevermore it will not mount an immune response to that protein ‘believeing’ that protein to be endogenous.

However it’s not perfect in all people and immune tolerance can be lost. Chimaerism has been implicated in many autoimmune diseases when the immune tolerance vanishes and the body starts attacking its own tissues.

I would just like to add that I predicted the suspect was a chimera well before he was even brought back and photographed. My brother and sister were shocked!!! That had never even heard of such a thing. And I solved the case with such a bazarre explanation well before it was presented in the show.

I’m the man.

Oo-oo-oo! golf clap Kudos! :cool:

Are you sure? Based on this thread, you could be the men! :smiley:

(I hope this isn’t horribly insensative to people who may be chimeras.)

ive been wanting to see if anyone can answer this question for a while. my family has a hereditary birth defect. but only for the females, its nothing horrible or even remotly bad. just raises questions. some how i am the only male in our family that has the abnormly short toe (secon to last next to the pinky toe on both feet). a long time ago my mother told me that i could of had a twin because there was a second heart beat or something like that, then there wasnt. this is aquestion thats been in my head about me since i was eleven. im twenty one now and i happened to come accross this. and im hopeing that some one could tell me how i ended up with a trait that only the females have in my family